A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is desirable to make measurements using an optical measuring technique. For example, with the aid of a lithographic apparatus, different patterns are successively imaged at a precisely aligned position onto the substrate. The substrate may undergo physical and chemical changes between the successive images that have been aligned with each other. The substrate is removed from the apparatus after it has been exposed with the image of at least one pattern, and, after it has undergone the desired process steps, the substrate is placed back in order to expose it with an image of a further pattern, and so forth, while it must be ensured that the images of the further pattern and any subsequent patterns are positioned accurately with respect to the at least one already exposed image on the substrate. To this end, the substrate is provided with alignment marks to provide a reference location on the substrate, and the lithographic apparatus is provided with an alignment system to measure the alignment position of the alignment marks. By measuring the alignment position of the alignment marks, in principle the position of every point on the substrate can be predicted, i.e., the location of a previously exposed target portion can be calculated and the lithographic apparatus can be controlled to expose a successive target portion on top of the previously exposed target portion.
Usually, the alignment marks on the substrate are diffraction structures such as diffraction gratings. The alignment system then comprises an alignment sensor system with a radiation source to emit radiation towards the grating and a detector to detect the diffraction pattern in the reflected radiation, i.e., sub-beams diffracted in a first, third and/or higher order are used, in order to determine the position of the grating.
Further, it is desirable to make measurements of the structures created (e.g., the device features in resist and/or other layer on or of the substrate), e.g., for process control and verification. One or more parameters of the structures are typically measured or determined, for example the overlay error between successive layers formed in or on the substrate. There are various techniques for making measurements of the microscopic structures formed in a lithographic process. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. An example of such a tool is a scatterometer developed for use in the lithographic field. This device directs a beam of radiation onto a target on the surface of the substrate and measures one or more properties of the redirected radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods, library searches, and principal component analysis. Like with alignment, the target may be a diffraction grating, e.g., typically a compound grating of a grating in one layer overlaid by another grating in another layer.